Kenji mentioned the gravity method of getting air out of a ziplock bag. However you can use a $30 pump and a bit of vegetable oil to get even better results. The following video shows you how.
Essentially this is the "straw" method of removing air, but you use a Q-tip to put a thin layer of oil an inch below the zipper. This oil barrier actually holds the vacuum instead of the ziplock.
You can also use this with the gravity method as well and get even better results because you can get the oil completely below the water line.